The production of titanium dioxide by the vapor phase oxidation of titanium tetrachloride has been known in the art for a long time and is generally described by the following reaction:TiCl4+O2→TiO2+2Cl2 It is known that this reaction can be initiated by heating the reactants (oxygen and titanium tetrachloride) to a suitable temperature. In a typical process, preheated titanium tetrachloride vapor and a preheated stoichiometric excess of oxygen are mixed at high flow rates at a pressure above atmospheric pressure in a tubular reactor (also referred to as an oxidizer) or series of such reactors wherein the titanium tetrachloride vapor reacts with the oxygen to produce solid titanium dioxide particles. The reaction of titanium tetrachloride vapors with oxygen to form titanium dioxide is exothermic. Methods and equipment for preheating the reactants are known in the art.
Typically, the reactors used for producing titanium dioxide have a generally tubular shape and at least a portion of the oxygen flow is introduced at one end of the reactor, forming an oxygen stream. The oxygen stream is transported through a conduit to a reaction chamber. The titanium tetrachloride is injected into the conduit through an injection slot at a point downstream from the end where the oxygen flow is introduced (that is, the upstream end) and upstream from the reaction chamber. Injection slots of the type conventionally used for titanium tetrachloride oxidation reactors may comprise, for example, a circumferential slot in the wall of the conduit, an arrangement of circumferential slots in the conduit wall, a separate injection chamber formed within the reactor, or an arrangement of separate injection chambers formed within the reactor.
One typical reactor utilized in the process for producing titanium dioxide, as described hereinabove, is shown in FIG. 1. In general, the reactor 100 comprises a first oxidizing gas introduction assembly 102 which receives preheated oxygen from oxygen preheat equipment 116 by way of a flowline 122 and introduces the oxygen through a conduit 128 and into a first reaction zone 104 formed in the reactor. The reactor 100 further comprises a first titanium tetrachloride vapor introduction assembly 106 which receives preheated titanium tetrachloride from titanium tetrachloride preheat equipment 118 by way of a flowline 124 and introduces titanium tetrachloride vapor through a first injection slot 108 into conduit 128. A second addition of preheated titanium tetrachloride may be made through a second titanium tetrachloride introduction assembly 110 receiving preheated titanium tetrachloride from titanium tetrachloride preheat equipment 120 by way of a flowline 126 and introducing the same through a second injection slot 112 into a second reaction zone 114. Secondary additions of preheated oxygen are also known, see, for example, U.S. Pat. No. 6,207,131 to Magyar et al. It will be appreciated that while first and second reaction zones 104 and 114 are described, the reaction between titanium tetrachloride and oxygen occurs in fact throughout the reactor downstream from an introduction of titanium tetrachloride and is not limited to any one particular reaction zone.
Typically, the titanium tetrachloride is preheated to a temperature from about 350° F. (177° C.) to about 1800° F. (982° C.) depending upon the particular preheat apparatus utilized. The oxygen is typically preheated to a temperature from about 1000° F. (538° C.) to about 2200° F. (1204° C.). First and subsequent additions, where employed, of titanium tetrachloride and/or oxygen may be at the same or different temperatures, see, for example, U.S. Pat. No. 6,387,347 to Deberry et al. (suggesting a secondary titanium tetrachloride addition at a lower temperature). The oxidation reaction temperature is typically from about 2300° F. (1260° C.) to about 2500° F. (1371° C.).
One of the most important aspects of oxidizer design concerns achieving an efficient mixing of the titanium tetrachloride and oxygen streams. In a typical reactor, efficient mixing generally requires that the titanium tetrachloride have a sufficient slot velocity as it passes through the slot and into the oxygen-carrying conduit. In typical oxidizers, such as the oxidizer 100 shown in FIG. 1, the slot size is fixed such that the pressure drop from the titanium tetrachloride vapor supply apparatus to the inside of the oxidizer is about 1 psi to about 2 psi. That is, the pressure at which the titanium tetrachloride is delivered (typically being from about 20 to 50 psig) is about 1 psi to about 2 psi higher than the pressure inside the oxidizer. This pressure drop will at typical titanium tetrachloride delivery pressures generally allow a slot velocity of about 200 ft/sec to about 300 ft/sec.
If the pressure at which the titanium tetrachloride is delivered decreases, the pressure drop and, thus the slot velocity, will also be reduced, resulting in less efficient mixing of the titanium tetrachloride and oxygen. A reduction in the titanium tetrachloride delivery pressure sometimes occurs as the result of a reduction in the flow rates from one or more chlorinators producing titanium tetrachloride in a production facility, for example, due to a shut down for maintenance or repair. Reduced titanium tetrachloride flow rates can also be seen as the result of fouling in one or more flowlines delivering titanium tetrachloride to the titanium tetrachloride supply apparatus.
To maintain efficient mixing of titanium tetrachloride and oxygen at reduced titanium tetrachloride flow rates, the slot velocity should be maintained. Maintaining slot velocity at reduced titanium tetrachloride volumetric flow rates requires the injection slot size to be reduced. Similarly, if the titanium tetrachloride flow rate increases then the size of the injection slot needs to be increased if the slot velocity is to be maintained. Thus, changing titanium tetrachloride flow rates requires a corresponding adjustment to the size of the titanium tetrachloride injection slot(s).
Those skilled in the art will appreciate as well, that where titanium tetrachloride flow rates are not changed but greater or lesser mixing is nevertheless desired for some reason (for controlling particle size of the produced titanium dioxide, for example), a corresponding change in slot velocity would likewise require a change in the size of the injection slot.
Generally in commercial reactors, however, the titanium tetrachloride injector will as indicated above have a fixed size injection slot through which the titanium tetrachloride is introduced into the reactor. This fixed size (or fixed “width”, as the slot size is typically described) injection slot must be adjusted manually in reactors of the prior art. In order to change the width of the injection slot in existing reactors, the oxidation process is first shut down. The reactor is then allowed to cool sufficiently so the reactor and/or injector can be taken apart and appropriate changes can be made. The reactor must then be reheated so that production can begin again. Thus, changing the width of the titanium tetrachloride injection slot can take several hours, during which production must be discontinued and costs incurred in making the changes and getting the reactor back online. Consequently, operators have tended to accept a certain degree of sub-optimum mixing in their oxidizers.
The present invention provides for novel apparatus and methods for injecting a first fluid flow into a conduit carrying a second fluid, wherein a desired degree of mixing can be maintained or established without the tradeoffs required in manually adjusting slot widths. In a first aspect, the present invention provides a fluid mixing apparatus comprising a fluid supply apparatus for supplying a first fluid; a conduit for transporting a second fluid; an adjustable injection slot providing fluidic communication between the fluid supply apparatus for the first fluid and the conduit carrying the second fluid; and a means for automatically or remotely adjusting the adjustable injection slot to maintain a substantially constant slot velocity or achieve a desired, new slot velocity. By “automatically or remotely adjusting” it is meant that the injection slot can be adjusted without manually disassembling the apparatus and adjusting the slot, as has been required previously. Further, where the slot is “remotely” adjusted, the adjustment is accomplished by operator intervention, whereas “automatic” adjustments are triggered and accomplished by a change in operating conditions such that an adjustment is indicated. A non-limiting example of means for “automatically adjusting” the slot width is provided below as a preferred embodiment of the present invention.
In a second aspect, the present invention provides a method for mixing two fluids, comprising the steps of: introducing a first fluid through an injection slot and into a conduit transporting a second fluid; and adjusting the injection slot width (whether automatically or remotely) while continuing to introduce the first fluid into the conduit.
The present invention has been advantageously utilized in a titanium dioxide production process. In a preferred embodiment, the present invention can be advantageously utilized to adjust the titanium tetrachloride injection slot width in a titanium dioxide production reactor while online, for example, to automatically compensate for changes in the flow rate of titanium tetrachloride and in so doing maintain a substantially constant slot velocity for continued good mixing characteristics and a desired pigment particle size, or on the other hand to alter the mixing characteristics and pigment particle size from the oxidizer through altering the slot velocity and associated mixing in the oxidizer.